1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more specifically to an air/fuel ratio control system designed for purifying exhaust gas on the basis of various operating parameters.
2. Description of Background Information
As an example of the system of this type for an internal combustion engine mounted on a vehicle, a technique is disclosed in Japanese Patent Publication No. 56-7508. In this technique, magnitude of the engine load is judged on the basis of detected results of various engine parameters such as the gear position, the vacuum level within the intake manifold, the vehicle speed, and the rotational speed of the engine. When the judged magnitude of engine load is within a predetermined load range, the system shifts the air/fuel ratio of the mixture to the lean side.
However, in this type of conventional air/fuel ratio control system, there has been a drawback that the engine output power becomes lower than a required value, causing the deterioration of the driveability of the vehicle under such a condition that the operating parameters change rapidly within a range for providing a lean air/fuel ratio due to a transition of the engine operation. This is because the air/fuel ratio is always made lean by this system when the magnitude of the engine load is within the range for providing the lean mixture without regard to state of the engine operation, i.e. whether it is under the transitional state mentioned above or under a stable state.